FIG. 5 is a sectional view showing a typical example of a method for joining oil well pipes. FIG.5(a) shows one type in which two pipes P.sub.1 and P.sub.2 are jointed to each other by use of a coupler C. In this type, a thread portion S, having a mail thread, is provided around each outer peripheral surface of the ends of the two pipes P.sub.1 and P.sub.2. The two pipes are joined together by screwing the male thread into the mating female thread provided around an inner peripheral surface of the coupler C. In the type shown in FIG.5 (b), a female thread is provided around an inner peripheral surface of one pipe P.sub.2, which is joined to another pipe P.sub.1 by screwing the female thread into the mating male thread provided around an outer peripheral surface of the another pipe P.sub.1.
In the types shown in FIG. 5, some pipes have a portion M (referred as a metal seal portion) corresponding to an end portion of the pipe to which no thread working is applied. There also exists a type, which has no such metal seal portion, but it is not shown in the drawing.
As described above, a thread working is applied to at least one of the outer peripheral surface of the end portions of the oil well pipes. Such a thread requires durability against repeated use, and a portion which is joined by the thread, also requires high air tightness. Thus, in an oil well pipe which has the thread worked portion with or without the metal seal portion, it is highly important to verify whether the worked portion of a pipe edge, including these portions, is ranged within a particular standardized dimension.
To cite an example for verification of the oil well pipe edge portion worked as shown in FIG. 5, there is one measurement method, in which the outer diameter, at a position of 2 mm from the pipe edge (the metal seal portion), is measured and the outer diameter, at one particular portion in the range of from 30 to 80 mm from the same pipe edge (the thread portion), is measured. The position in the latter measurement is varied according to a size of an oil well pipe. In the conventional method, when the outer diameter is measured at such plural designated positions, stand off ring gauges, which have respectively different diameters corresponding to each prescribed outer diameter at the plural designated positions, have been prepared for each the pipe size. However, this conventional method has the following enumerated drawbacks.
(1) A number of gauges need to be prepared for each pipe size and also for each measuring position. Therefore, the expenditure for their fabrication would run up and this also takes a lot of trouble due to the checking operation for quality control of the gauges.
(2) Since it needs to perform the measurement with fitting the gauge onto the measuring position from the outside, this occasionally causes to make some defects on the peripheral of the pipe.
(3) In a case that the thread worked portion is out of the complete round, the measurement error would be increased.
There also exists a method for directly measuring the outer diameter dimension of the designated portion besides the aforementioned method. However, such a method has the following drawbacks.
(4) Since the maximum size of the outer diameter is measured by making a reference portion of the gauge contact at the pipe end, it needs to turn the gauge around with making the reference portion serve as a fulcrum. Accordingly, the measurement attitude could become unstable, and this tends to cause measurement error.
(5) Since the outer diameter of the designated portion is measured at several points and these sizes are visually read, a continuous and rapid measurement cannot be carried out.
(6) Since plural designated portions need to be measured with respective particular gauges, the designated portions cannot simultaneously be measured.
As described above, any conventional measuring methods are inefficiently performed by manual procedures, and it is also difficult to measure with high accuracy.